This invention relates to a stator structure of the type used in rotary machines, and more specifically, to structure within the compression section to guide working medium gases through the section.
An axial flow rotary machine, such as a gas turbine engine for an aircraft, has a compression section, a combustion section, and a turbine section. An annular flow path for working medium gases extends axially through the sections of the engine. The gases are compressed in the compression section to raise their temperature and pressure. Fuel is burned with the working medium gases in the combustion section to further increase the temperature of the hot, pressurized gases. The hot, working medium gases are expanded through the turbine section to produce thrust and to extract energy as rotational work from the gases. The rotational work is transferred to the compression section to raise the pressure of the incoming gases.
The compression section and turbine section have a rotor which extends axially through the engine. The rotor is disposed about an axis of rotation Ar. The rotor includes arrays of rotor blades which transfer rotational work between the rotor and the hot working medium gases. Each rotor blade has an airfoil for this purpose which extends outwardly across the working medium flow path. The working medium gases are directed through the airfoils. The airfoils in the turbine section receive energy from the working medium gases and drive the rotor at high speeds about an axis of rotation. The airfoils in the compression section transfer this energy to the working medium gases to compress the gases as the airfoils are driven about the axis of rotation by the rotor.
The engine includes a stator disposed about the rotor. The stator has an outer case and arrays of stator vanes which extend inwardly across the working medium flowpath. The stator extends circumferentially about the working medium flow path to bound the flow path. The stator includes an outer flowpath wall (outer case) and seal elements supported from the wall for this purpose. An example is an inner shroud assembly having a circumferentially extending seal member (rubstrip). The rubstrip is disposed radially about rotating structure and may be supported, for example, by the vanes through an inner shroud. The rubstrip is in close proximity to associated knife-edge seal elements which extend circumferentially on the rotor and together form a seal that blocks the leakage of working medium gases from the flowpath.
The stator vanes and the rotor blades are designed to receive, interact with and discharge the working medium gases as the gases are flowed through the engine. The arrays of stator vanes are disposed upstream of the arrays of rotor blades in both the compression section and turbine section. The stator vanes each have an airfoil located in a predetermined manner with respect to the adjacent stator vanes for guiding the working medium gases to the rotor blades. The airfoils in the forward portion of the compression section are frequently struck by foreign objects that flow into the engine with the incoming stream of gases. These may include large foreign objects such as wild fowl or chunks of ice that may break away from adjacent structure under operative conditions. The stator vane immediately downstream of the fan blade must tolerate these impacts without tearing loose from adjacent structure and moving rearwardly into the adjacent stage of rotating rotor blades. In addition, the stator vanes are frequently replaced over the life of the engine. The replacement vanes are preferably located in a repeatable fashion such that the aerodynamic characteristics of the array of compressor vanes are maintained. Finally, seal elements such as rubstrips formed of silicone rubber which are supported by the stator vanes must tolerate severe rubs from rotating structure. Such rubs may occur during normal operative conditions of the engine or during abnormal operative conditions that might occur, for example, after an impact by a foreign object against the engine. The rubstrip must tolerate the severe rub without delaminating (a noncohesive failure) and moving into the flow path and the stator vane must have a large enough base to support the airfoil and seal against such rubs.
Another concern is the manufacture of the stator vane from a vane blank. The present designs of the stator vane blank, as shown in the prior art drawings 3A-3C, results in forming a rather large pedestal on the inner surface of the base of the vane. The mass of the pedestal both contributes to engine weight and to possible disruption of the engine flowpath. Nevertheless, the mass of material on the forged head of the vane blank is required for both the forging operation and, in some cases, for the machining operation.
The above notwithstanding, scientists and engineers working under the direction of Applicants Assignee have sought to develop stator assemblies having arrays of stator vanes that are locatable in repeatable fashion after replacement and have acceptable levels of durability and replaceability.
According to the present invention, a stator vane blank for forming a stator vane for a rotary machine has a forged head having a pair of angled surfaces which form the outer surface that are at a greater angle than the angle between the pair of angled surfaces which form the inner surface that faces toward the airfoil.
In accordance with the present invention, the outer surface has an included angle alpha at the outer surface which is about nine degrees and an included angle beta at the inner surface which is about three degrees or less. In one embodiment, the angled surface has an included angle beta that is about one and one-half (1xc2xd) degrees
A primary feature the present invention is the included angle alpha at the outer surface after the vane blank is forged. Another feature is the included angle beta at the inner surface after the vane blank is forged. Another feature is the mass of material on the outer surface of the vane blank as compared to the smaller mass material on the inner surface of the vane blank where the overall mass of material is a required minimum amount for forging and machining operations. Another feature is the size of the pedestal after the inner surface is machined to form the inner surface of the stator vane by reason of the angle beta being smaller than the angle alpha and thereby creating a smaller pedestal because of the distance from the inner surface to the inner most portion of the head on the vane blank.
A primary advantage of the present invention is the cost of machining a stator vane from a forged vane blank where the stator vane has less material disposed on its inner surface that must be machined away by reason of the included angle beta being smaller than the included angle alpha on the outer surface. Another advantage is the engine efficiency which results from the weight of the engine which weight is reduced by having a smaller pedestal left behind after the inner surface is finally machined by disposing the mass of material that is required for the head during forging and machining operation on the outer surface where the material is machined away in the final machining process.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of the best mode for carrying out the invention and accompanying drawings.